su can be used for this purpose. If you want to become user bob you would type:

su bob

su will then ask for a password, which is bob's password, not yours. When invoked without arguments it tries to become superuser (same as su root). You may have to add your user account to the wheel group, or change some configuration so that the normal user can use su.

I usually do:

sudo su - bob

so I won't need to know the password of the other user (Note: I'm a Linux user and I'm not sure how different BSD is).